From 2dc9c23d07864f9d544ac778d732ebd81d44c404 Mon Sep 17 00:00:00 2001
From: Robert Habermeier <rphmeier@gmail.com>
Date: Mon, 9 Jan 2017 14:56:06 +0100
Subject: [PATCH] fix deadlock in queue drop (#4095)

Former-commit-id: 923fc16078834f0c63e245dcb01ee95210bfc7f7
---
 ethcore/src/verification/queue/mod.rs | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/ethcore/src/verification/queue/mod.rs b/ethcore/src/verification/queue/mod.rs
index d2f96ea80..007c5de92 100644
--- a/ethcore/src/verification/queue/mod.rs
+++ b/ethcore/src/verification/queue/mod.rs
@@ -97,7 +97,6 @@ pub struct VerificationQueue<K: Kind> {
 	engine: Arc<Engine>,
 	more_to_verify: Arc<SCondvar>,
 	verification: Arc<Verification<K>>,
-	verifiers: Vec<JoinHandle<()>>,
 	deleting: Arc<AtomicBool>,
 	ready_signal: Arc<QueueSignal>,
 	empty: Arc<SCondvar>,
@@ -105,6 +104,7 @@ pub struct VerificationQueue<K: Kind> {
 	max_queue_size: usize,
 	max_mem_use: usize,
 	total_difficulty: RwLock<U256>,
+	verifier_handles: Vec<JoinHandle<()>>,
 }
 
 struct QueueSignal {
@@ -186,7 +186,7 @@ impl<K: Kind> VerificationQueue<K> {
 		let empty = Arc::new(SCondvar::new());
 		let panic_handler = PanicHandler::new_in_arc();
 
-		let mut verifiers: Vec<JoinHandle<()>> = Vec::new();
+		let mut verifier_handles: Vec<JoinHandle<()>> = Vec::new();
 		let thread_count = max(::num_cpus::get(), 3) - 2;
 		for i in 0..thread_count {
 			let verification = verification.clone();
@@ -196,7 +196,7 @@ impl<K: Kind> VerificationQueue<K> {
 			let empty = empty.clone();
 			let deleting = deleting.clone();
 			let panic_handler = panic_handler.clone();
-			verifiers.push(
+			verifier_handles.push(
 				thread::Builder::new()
 				.name(format!("Verifier #{}", i))
 				.spawn(move || {
@@ -213,13 +213,13 @@ impl<K: Kind> VerificationQueue<K> {
 			ready_signal: ready_signal.clone(),
 			more_to_verify: more_to_verify.clone(),
 			verification: verification.clone(),
-			verifiers: verifiers,
 			deleting: deleting.clone(),
 			processing: RwLock::new(HashMap::new()),
 			empty: empty.clone(),
 			max_queue_size: max(config.max_queue_size, MIN_QUEUE_LIMIT),
 			max_mem_use: max(config.max_mem_use, MIN_MEM_LIMIT),
 			total_difficulty: RwLock::new(0.into()),
+			verifier_handles: verifier_handles,
 		}
 	}
 
@@ -531,8 +531,15 @@ impl<K: Kind> Drop for VerificationQueue<K> {
 		trace!(target: "shutdown", "[VerificationQueue] Closing...");
 		self.clear();
 		self.deleting.store(true, AtomicOrdering::Release);
-		self.more_to_verify.notify_all();
-		for t in self.verifiers.drain(..) {
+
+		// acquire this lock to force threads to reach the waiting point
+		// if they're in-between the exit check and the more_to_verify wait.
+		{
+			let _more = self.verification.more_to_verify.lock().unwrap();
+			self.more_to_verify.notify_all();
+		}
+
+		for t in self.verifier_handles.drain(..) {
 			t.join().unwrap();
 		}
 		trace!(target: "shutdown", "[VerificationQueue] Closed.");